A copending application U.S. Ser. No. 216,876, filed Dec. 15, 1980, now U.S. Pat. No. 4,355,265, describes a lamp starting circuit including an input ballast circuit that typically comprises an inductor, capacitor and resistor, and a circuit shorting switch, referred to therein as a glow-starter device. The circuit shorting switch operates in a controlled gas atmosphere as described in copending application U.S. Ser. No. 216,874, filed Dec. 15, 1980 now U.S. Pat. No. 4,329,621. The shorting switch starter is described therein as having a first terminal connected to a first bimetal, a second terminal connected to a second bimetal, and a third terminal connected to a rigid conductive member, the bimetals being electrically connected together at one end which makes a normally closed contact with the rigid member in the quiescent state of the device; means connecting the second and third terminals of the starter device across the output of the ballast circuit; and means for connecting the first and third terminals of the starter device across the terminals of a discharge lamp. Upon initial energization of the ballast, short circuit current through the second and third terminals of the starter device is operative to flex the second bimetal for separating the bimetals from the rigid member to provide an open circuit thereat and a switching transient across the lamp. Upon occurrence of this open circuit, the lamp is adapted to start. Lamp current flowing through both bimetals maintains the switch open and the lamp lit.
The transient voltage resulting from the opening of the shorting switch is equal to L di/dt. L is the inductance of the reactor; i is the current flowing at the instant the switch opens; and t is the time for the switch to open. If the switching transient voltage is sufficiently high, lamp ignition into the glow state results.
The open circuit steady state voltage (OCV) applied across the lamp immediately after the switching transient is the summation of the line voltage and the voltage across the capacitor of the ballast circuit at the instant the switch opens, or, OCV=V.sub.c +V input. Thus, the open circuit voltage is the input AC voltage displaced from a zero reference by the DC capacitor voltage. Thus, the peak value of the open circuit voltage is the peak of the input voltage in the direction of the capacitor voltage plus the capacitor voltage itself. Because the capacitor voltage wave shape resembles a sine wave with its top flattened, there is a good probability that the voltage across the capacitor is somewhat near its peak after the switch opens. When the capacitor voltage is sufficiently high, the open circuit voltage is large enough to initiate the transition from the glow to the arc state. Simultaneously, the energy stored in the ballast capacitor (1/2 CE.sup.2) is discharged through the lamp. If there is insufficient energy the arc extinguishes. Therefore, sufficient energy must be presented to the discharge to ensure arc sustainability.
In the aforementioned copending application U.S. Ser. No. 216,874, the three terminal bimetal starter switch is sealed in a controlled gas atmosphere to control the amplitude of the switching transient voltage. The controlled gas atmosphere may be an argon gas fill. The control of the amplitude of the transient voltage is necessary to ensure that excessively large voltages are not generated which might otherwise cause a component failure. In the aforementioned copending application U.S. Ser. No. 216,876, the circuit described therein with the use of a gas atmosphere enclosing the switch did indeed reduce the maximum transient voltage. However, it was also found that the gas atmosphere switch provided a path in parallel with the discharge lamp through which the capacitor would at least partially discharge. Thus, a significant portion of the capacitor energy, intended to be transmitted to the lamp for glow-to-arc transition, was being dissipated in the gas switch. This resulted in numerous arc extinguishments and overall less reliable starting.